The present invention relates to dyeing of cross-linked phenolic resin fibers and blends thereof, using anionic or premetallized dyes.
Novoloids, infusible cured phenolic resins containing at least 85 percent cross-linked novolac, may be produced by fiberization of a novolac melt and subsequent cross-linking or curing to an infusible stage. Curing may be effected in the presence of a source of methylene groups, such as paraformaldehyde, formaldehyde, or hexamethylenetetramine, and preferably also in the presence of an acidic or basic catalyst. Such novoloid fibers are disclosed in the following U.S. Patents, which are incorporated herein by reference: U.S. Pat. No. 3,650,102, issued Mar. 21, 1972, to Economy et al.; U.S. Pat. No. 3,716,521, issued Feb. 13, 1973, to Economy et al.; and U.S. Pat. No. 3,723,588, issued Mar. 27, 1973, to Economy et al. Novoloid fibers may be formed into felts, mats, cloths, rovings, or other useful embodiments in accordance with conventional fiber handling techniques. A fabric of such fibers is disclosed by U.S. Pat. No. 3,628,995, issued Dec. 21, 1971, to Economy et al.
Novoloid fibers have a number of highly desirable attributes which render them of value in numerous applications. Their most outstanding virtue is excellent flame resistance. When subjected to flame, the infusible fibers do not melt, but rather char to produce carbon fibers, which continue to retain the shape and approximate dimensions of the original fibers, and which continue to afford extremely effective protection from flames. Accordingly, the fibers are of great utility in the fabrication of flame-protective clothing, as well as drapes, carpeting, upholstery, and the like which are especially suited for use in areas where fire constitutes a particular hazard. Such fibers also provide very effective thermal and accoustical insulation, and again are particularly useful for these applications where fire is a hazard. The fibers have suitable mechanical properties, such as tenacity and break elongation, to permit their being processed into yarns, woven and knitted fabrics and the like, as well as various non-woven forms such as felt, batting, and paper.
Notwithstanding such desirable attributes, infusible cured phenolic resin fibers have several disadvantages. Just after curing, they are generally quite intensely colored, the hue ranging from very pale yellow to gold. Moreover, upon standing, the coloration may increase considerably in intensity, becoming deep brown or reddish brown. Thus, the fibers are known to possess rather poor color fastness.
Such novoloid fibers are particularly resistant to normal commercial dyeing processes. Accordingly, it has been difficult to obtain fibers and/or fabrics of the desired range of colors and color fastness. This detriment has a marked effect upon their acceptance by the textile industry and by the consumer with respect to applications for such fibers and fabrics in which color is an important factor.
In an attempt to overcome the deep coloration of such fibers, esterification or etherification of the phenolic hydroxyl groups may be utilized. In accordance with U.S. Pat. No. 3,716,521, of Economy et al., infusible cured phenolic resin fibers may be reacted with any of a wide variety of suitable esterification or etherification reagents, at a suitable temperature, for sufficient time to block at least about 50 percent, and preferably about 90 percent, of the phenolic hydroxyl groups of the cured resin. The blocking of phenolic hydroxyl groups may be accomplished after the resin has been cured to the point of infusibility, notwithstanding the cross-linked nature of the resin. Such fibers, while generally white in color and quite colorfast, have also, in the past, been difficult to dye to desired shades using commercially acceptable dyeing processes.
Acid, or anionic dyes, contain as the active principle aromatic compounds including in their structure both a chromophoric group and a water-solubilizing group. Usually the solubilizing group is the sulfonic acid radical, --SO.sub.3 H. The commercial dyes appear in the form of the sodium salt, normally diluted to a standard effective concentration with anhydrous sodium sulfate. Typical chemical structures found in anionic dyes include nitro, monoazo, diazo, triphenyl-methane, anthraquinone, azine, and quinoline.
One class of anionic dyes is the premetallized acid dye. These dyes may be 1:1 or 1:2 metal complex dyes, containing one metallic atom for each one or two dye molecules. Usually the dye molecule is an azo type or an azomethine, while the metal in most instances is chromium, cobalt, or iron. Both anionic and premetallized dyes have been successfully used for dyeing wool and silk fibers, giving excellent fastness. However, they have not been successfully used for dyeing phenolic fibers, and novoloid fibers in particular, prior to the present invention.